In the foundry art, sand casting is used to make metal parts. In sand casting, foundry shapes called molds, exterior casting forms, and cores, interior casting forms, are made from a mixture of a foundry aggregate, such as sand, and a binder. The two categories of sand molds are “green” and “rigid.” Green sand molds are bonded with clay and water. Rigid sand molds are bonded with organic resins and can be hardened using one of several methods, including baking, curing with a chemical reagent, and flushing with a reactive gas. Molten metal is poured into and around the foundry shapes after they have hardened. The binders, e.g., phenol formaldehyde resins, or phenol-isocyanate (polyurethane) resins, used to form foundry shapes typically contain a significant amount of organic solvent, which can emit noxious fumes, as well as free reactants, e.g., formaldehyde or isocyanate and free phenol, that are detrimental to the cast metal and the foundry environment.
A urethane binder resin, when used in combination with a foundry aggregate such as sand, typically has three parts: a polymerizable polyol component, an isocyanate component, and a catalyst component. The polymerizable polyol compound, e.g., a polyol, such as ethylene glycol, and isocyanate react to form a cross-linked polymer which increases the tensile strength and/or hardness of the sand/binder mixture. The catalyst prompts the polymerizable components of the binder to polymerize, allowing for rapid conversion of the foundry mix into a hard and solid cured state suitable for forming shapes from molten metal poured therein.
The addition of other polyols to urethane based resin systems has been recently illustrated by Thiel in WO 2009/065018 A1. Therein, Thiel suggests that humic substances represent an alternative reactant to the conventional polyol component in reactions with isocyanates for the formation of binder resins. Generally, humic substances, e.g., humic acid, fulvic acid, hymatomelanic acid, ulmic acid, and humin, contain hydroxyl groups that are thought to react with an isocyanate to form polyurethane resins.
Humic acid has previously been included in foundry sand compositions, i.e. green sands. See U.S. Pat. Nos. 3,023,113 and 3,832,191. Humic acid is obtainable from multiple natural sources, including lignite, leonardite, peat, and manure. Lignite and leonardite are rich in humic acid and are easily accessible by mining. Lignite is an organic mineraloid that is the lowest rank of coal. Known as “brown coal,” lignite has a high inherent moisture content of up to 66 percent and a high ash content compared to other forms of coal. Lignite has previously been used as an additive in foundry sand compositions, i.e. green sands, comprising an additional binding agent such as bentonite clay. See U.S. Pat. Nos. 3,445,251 and 4,359,339.
U.S. Pat. Nos. 3,023,113, 3,832,191, 3,445,251 and 4,359,339 teach the addition of humic substances to foundry aggregates in green sand applications. These patents teach that when the humic substance is mixed with a clay binder foundry shape performance is improved.
A humic substance-based urethane resin mix, described by Thiel, for use as a foundry sand binder was found to have a short shelf life, with gross separation (settling) of humic solids and/or to turn into a stiff (i.e. no fluidity) gel within a short duration of storage. The separation of the humic solid prevents the ready standardization of the addition of the humic solids to foundry aggregate over the course of normal foundry mold and core production. These variations in the composition of the foundry mix can affect casting performance of the foundry shape thereby impacting the cost and quality of the resultant casting in a negative manner.
No patent or application teaches the direct addition of humic solids to foundry aggregates in rigid sand mold applications. Moreover, there is no teaching that one can provide a consistent and repeatable amount of humic solids to the foundry mix by providing direct addition of the humic solid to the foundry aggregate. The resin components and molding described herein include a humic substance, e.g., a lignite, e.g., leonardite and a polyol-isocyanate resin binder, wherein a solid lignite containing up to about 25 wt. % water, based on the dry weight of the lignite, is mixed into the foundry aggregate. The lignite component is mixed into the foundry aggregate as a solid having a long shelf life for convenient use at a foundry. It has been found that improved properties result from mixing the lignite in dry form (less than 15% moisture) into the foundry sand with separate addition of polyol, isocyanate and catalyst.